It is known that products made from polyamides are sometimes subject to marked degradation from exposure to light and heat, resulting in poorer mechanical properties, such as reduced hardness, tenacity and elasticity, and in rigidity and fragility. Degradation may cause yellowing of the product, if due to the light, or even produce a brown color, if due to heat. Such color changes too are undesirable and harmful.
The art believes said degradation to be largely due to oxidation caused by peroxide radicals, formed by the action of light or heat, which convey atmospheric oxygen to the polymer chain structure.
To prevent degradation, oxidation inhibitors having the property of capturing radicals are usually added to the polyamides. Generally said inhibitors were previously known as oxidation inhibitors for rubber, lubricating oils, etc.
Among them, the most commonly used are phenol or arylamine compounds and compounds containing halogens, heavy metals, or phosphorus.
The phenol oxidation inhibitors are alkylated phenols which confer a certain stability to light and often avoid undesirable discoloration, but are generally not effective against heat a temperatures above 165.degree. C.
The arylamine oxidation inhibitors, used with success in the rubber and lubricating-oils industry, have the drawback that when used to stabilize polyamides, they produce colors from yellow to brown and are ineffective against light.
The phosphorus oxidation inhibitors, which may be alkali or alkali-earth salts or amino compounds of phosphorus containing acids, such as hypophosphites, phosphites and phosphates, or their organic derivatives such as arylphosphites and arylhypophosphites, do not cause discoloration but are less effective stabilizers than arylamines, and sometimes affect the rheologic properties of the polymers and therefore the drawability of fibers made therefrom.
The halogenated oxidation inhibitors, which are mostly halogenated organic compounds or preferably alkali or alkaline-earth metal or amino halides, have the same drawbacks as the phosphorus compounds.
It is also known to employ mixtures of this last type of inhibitors with phenol, arylamino or phosphorus inhibitors, to obtain a balanced oxidation inhibition while leaving the polymer's properties unaffected, insofar as possible. The results thus obtained are sometimes satisfactory for molded polymers, but not for the production of synthetic fibers.
Further complementary agents are used to boost the action of oxidation inhibitors, which contain heavy metals in the form of inorganic or organic salts. In the case of polyamides, zinc, chromium, manganese, copper, tin, and germanium salts are mostly used. When associated with the aforementioned oxidation inhibitors they may also facilitate the molecular dispersion of the polymer and promote synergistic actions due to the ability of the several inhibitors to act at different temperatures.
Zinc, chromium, tin and germanium salts, however, promote heat stability only to a limited extent. Manganese salts are light stabilizers as effective as copper salts, but are ineffective against heat, and sometimes confer undesirable coloration to the polymer.
Copper salts have undesirable pink, red, or violet colors in the polymers due to the precipitation of copper in colloidal form during the polymerization and especially during the re-melting of the polymer granules for spinning or molding. They have a certain usefulness when associated with phosphorus compounds, such as phosphoric or phenylphosphorous acids, but their drawbacks remain, and the polymer viscosity is adversely affected by the phosphorus compounds. Furthermore the bivalent copper salts oxidize the polymers and are reduced to univalent salts.
Univalent copper halides, while effective light and heat stabilizers for polyamides are only used to a very limited extent since they are insoluble in water and can be kept in solution only by using a great excess of halogenic acids.
The reaction EQU CuX+HX.revreaction.[CuX.sub.2 ].sup.- H.sup.+ ( 1)
is an equilibrium reaction and at least 10 mols of halogen per mol of copper are required to shift it in the direction required for maintaining the CuX in solution. Since preferably between about 0.6-1.0 mols of copper per ton of polymer are used, 10 or more mols of halogenic acid per ton of polymer would be necessary, besides the water pertaining thereto, and such large amounts of halogen are very undesirable.